Photovoltaic devices utilize specific conductivity characteristics of materials generally referred to as semiconductors, whereby solar energy or radiation is converted to useful electrical energy. This conversion results from the absorption of photon energy in the active region of the cell, whereby some of the absorbed energy causes the generation of electron-hole pairs. The energy required for the generation of electron-hole pairs in a semiconductor material is referred to as the band gap energy and generally is the minimum energy required to excite an electron from the valence band to the conduction band.
There are two principal measures of the utility of photovoltaic devices. First is the efficiency of the device, which is an ascertainable percentage of the total photon energy converted to useful electrical energy. High efficiency photovoltaic devices made of crystalline materials maximize efficiency by minimizing internal lattice defects. The second measure of the utility of a photovoltaic device is its cost. Single crystal devices are complex and costly to produce, and do not readily lend themselves to high volume production.
One approach to reducing the cost of photovoltaic devices is to utilize polycrystalline thin film materials and a heterojunction. A heterojunction refers to the active junction formed at the interface between two dissimilar materials, such as cadmium sulfide and cadmium telluride, as taught by Basol, et al. in U.S. Pat. No. 4,388,483. Basol, et al. described thin-film heterojunction photovoltaic cells wherein the active layer comprises at least one of the metal elements of Class IIB of the Periodic Table of Elements and one of the non-metal elements of Class VIA of the Periodic Table of Elements. One feature of such photovoltaic devices is the use of extremely thin active layers. As an example, Basol, et al. utilized a cadmium sulfide layer on the order of 0.02-0.05 micrometers and a cadmium telluride layer on the order of about 1.3 micrometers. While such economy of material has obvious advantages, it has also presented an unexpected concern with respect to current collection.
Such thin-film photovoltaic devices typically comprise an optically transparent substrate, a first semiconductor layer formed on the transparent substrate, a second semiconductor layer of opposite conductivity type than the first semiconductor layer forming a junction with the first layer, and a back contact comprised of a conductive film. When the substrate is not electrically conductive, there is disposed between the substrate and the first semiconductor layer a front contact which is a transparent conductive layer, such as a transparent conductive oxide. This layer functions as a current collector for the photovoltaic device. Transparent conductive oxides, such as indium tin oxide, indium oxide, tin oxide, and zinc oxide are not efficient current collectors in cells of any apprecicable size, that is greater than about one square centimeter, due to their inherent sheet resistivity of about 10 ohms per square. The transparent conductive layer must be supplemented with more efficient current collection means. Formation of front contact current collector means for thin-film photovoltaic devices presents novel concerns as the general thickness of a front contact grid-type current collector disposed in contact with the transparent conductive layer typically exceeds the combined thickness of the first semiconductor layer and extends into the second active layer. This geometry creates problems with respect to shorting of the device and to uniform formation of the semiconductor layers themselves.
It is one object of the present invention to provide a front contact current collector grid for photovoltaic devices, wherein the thickness of the current collector grid exceeds the thickness of the first semiconductor layer, which does not suffer from the above-identified shortcomings.
It is another object of the present invention to provide materials suitable for use as current collector grids in photovoltaic device wherein the thickness of the current collector grid exceeds the thickness of the first semiconductor layer.
It is yet another object of the present invention to provide a method for the formation of a current collector grid in a photovoltaic device, which grid has a thickness exceeding the thickness of the first semiconductor layer.
These and additional objects of the present invention will become apparent to one skilled in the art from the below description of the invention and the appended claims.